Lithography is a printing method employing the characteristics in which oil is immiscible with water. On a planographic printing plate are formed image portions, which repel water and receive oleophilic ink, and non-image portions, which repel oleophilic ink and receive water. Then, lithographic printing is carried out in which only oleophilic ink received on the image portions of such a planographic printing plate is transferred to a paper sheet. As a printing press for lithographic printing is generally used an off-set printing press in which the oleophilic ink received on the image portions is once transferred onto a blanket, and then the ink on the blanket is further transferred onto a paper sheet.
A light sensitive planographic printing plate precursor comprises a support and provided thereon, a light sensitive layer. As the support of the light sensitive planographic printing plate precursor, one having high hydrophilicity, high water retention property, and excellent adhesion to the light sensitive layer is desired, and an aluminum plate whose surface is subjected to roughening treatment is generally used.
As a method for surface roughening a support is known a mechanical surface-roughening method such as a ball graining method, a brush graining method, a blast graining method, a buffing method or a horning method, or a electrochemical method in which the support surface is electrically roughened in an electrolyte solution of an acid such as hydrochloric acid or nitric acid through a direct current or an alternating current. The surface of an aluminum plate treated according to the method described above is relatively soft and is likely to be abraded, the surface-roughened aluminum plate is subjected to anodization treatment to form an anodization film on the surface. The resulting aluminum plate surface is hard and excellent in abrasion resistance.
An exposed planographic printing plate precursor is developed with an alkaline aqueous solution to remove a light sensitive layer at exposed or unexposed portions and unveil the support surface. The support surface is generally hydrophilic, and the support surface unveiled by development (corresponding to non-image portions) receives water and repels oil-based ink. In contrast, portions (corresponding to image portions), in which the light sensitive layer has not been removed by development, are generally oleophilic, and receive oil-based ink and repel water.
A light sensitive planographic printing plate precursor with high sensitivity reduces the time required for exposure, and can shorten the time necessary to prepare a printing plate. When printing is carried out employing a printing plate obtained from such a planographic printing plate precursor, printing is often suspended on account of registering or a break. When printing, after suspended, is resumed, minute stain spots (hereinafter referred to as peppering) may occur at the non-image portions of the printing plate. In order to prevent occurrence of the peppering, a method has been desired which maintains hydrophilicity of the non-image portions of the printing plate.
An aqueous alkali metal silicate solution has been widely used as a developer of the light sensitive planographic printing plate precursor. The alkali metal silicate of the aqueous alkali metal silicate solution reacts with the aluminum plate support, whereby hydrophilicity of the aluminum support surface is increased. However, this method alone cannot provide sufficient hydrophilicity of the support surface and cannot completely prevent occurrence of the peppering.
A photopolymerization type light sensitive planographic printing plate precursor comprising a photoinitiator and an ethylenically unsaturated monomer, which is capable of forming a latent image due to free radical reaction and developed with an aqueous alkali solution to form an image, has high sensitivity. The photopolymerization type light sensitive planographic printing plate precursor generally has high sensitivity, but has problem in that free radical reaction is inhibited by oxygen. In order to overcome the problem, an oxygen-shielding layer is provided on the light sensitive layer. It is well known that a layer containing polyvinyl alcohol as a main component is effective as the oxygen-shielding layer. However, the polyvinyl alcohol-containing layer, when incorporated in a developer during development, produces sludge in the developer. In order to overcome this problem, it is effective that washing (pre-washing or washing before development) for removing the oxygen-shielding layer is carried out employing a pre-washing solution prior to development. However, it has been found that when a light sensitive planographic printing plate precursor comprising the polyvinyl alcohol-containing layer, is repeatedly subjected to processing as described above, employing an automatic developing machine, sludge occurs in a pre-washing solution, resulting in problem requiring an additional work such as cleaning or unclogging of a pump. As the countermeasure, a method is disclosed in Japanese Patent O.P.I. Publication No. 10-10754 in which various additives are added to a pre-washing solution for pre-washing prior to development. However, this method cannot solve the problems including occurrence of the peppering.